Many studies of organic semiconductor materials have been made since electric conductivity of an organic compound that was subjected to a predetermined doping process was known to be similar to that of copper in the year 1970s.
In recent years organic semiconductor compounds have been frequently applied to organic electronic devices such as organic thin film transistors (OTFT), organic light-emitting diodes (OLED), organic solar cells, an organic laser, an electromagnetic wave blocking film, capacitors, and memory devices in consideration of properties such as electric conductivity, lightness in weight, flexibility, and processability of the organic semiconductor compound.
A thin film transistor that is a kind of field-effect transistor (FET) is a basic structure used in the electronic engineering field, and extensively used as a switching display device such as a liquid crystal display. Most thin film transistor is produced by using polysilicon or amorphous silicon as a semiconductor substance. The production process using silicon includes a high temperature process, a high vacuum system process, and a patterning process using a complicated photolithography method.
However, in a process of producing an organic thin film transistor using an organic substance, a relatively low temperature process is performed and a thin film is formed by using a solution process, thus the thin film transistor is produced by using a simple process at low cost. Furthermore, since the organic thin film transistor using the organic substance is compatible with a plastic substrate, it is light in weight and capable of being used in flexible goods.
The organic thin film transistor includes a gate electrode, an insulating layer, source and drain electrodes, and an organic semiconductor layer. The amount of current between the source and the drain is controlled by voltage applied to the gate.
In respects to an operation mechanism of the organic thin film transistor, operation of a P-type organic semiconductor will be described. If positive voltage is applied to a gate, a negative charge is induced at an interface between an organic semiconductor and an insulating layer. In the case of the P-type organic semiconductor, since there is a significant difference between a LUMO level (lowest unoccupied molecular orbital level) and a Fermi level of an electrode, it is difficult to inject electrons to the organic semiconductor. Therefore, even if voltage is applied between the source and the drain, a current does almost not flow. In contrast, if negative voltage is applied to the gate, an accumulation layer into which a positive charge is induced is formed in the vicinity of the insulating layer due to electric field induction resulting from the applied voltage. Generally, since a HOMO level (highest occupied molecular orbital level) of the P-type organic semiconductor is similar to the Fermi level of the electrode, the positive charge may be easily injected from the electrode to the organic semiconductor. Since there are many conductible charge carriers between the source and the drain, the current may desirably flow. In this connection, a drain current is increased in proportion to a drain voltage. Furthermore, if the drain voltage is desirably increased, an electric potential is rapidly changed at the outskirts of the drain to form a depletion layer of electrons and saturate the drain current. Hence, even though the drain voltage is increased, the drain current is maintained.
The organic thin film transistor is evaluated by using performance indexes such as field-effect mobility (μFET, □/V□sec), an on/off current ratio (Ion/Ioff), a threshold voltage (VT), a sub-threshold slope (SS, V/dec). The field-effect mobility means a speed (□/sec) of a charge in a unit electric field of 1 V/□ and relates to an operation speed of the transistor. This value is calculated by using a correlation graph of the drain current and the gate voltage. In a saturation state of the drain-source current, the field-effect mobility is calculated by using the following equation.
                              I          DS                =                                            WC              i                                      2              ⁢                                                          ⁢              L                                ⁢                                                    μ                FET                            ⁡                              (                                                      V                    G                                    -                                      V                    T                                                  )                                      2                                              <        Equation        >            
The above-mentioned equation is used to obtain the field-effect mobility in the saturation state of the drain-source current. IDS means the drain-source current in the saturation state, μFET means the field-effect mobility, Ci means a capacitance per unit area of a gate insulator, VG means a gate voltage, and VT means a threshold voltage. In the equation, the performance of the organic thin film transistor may vary according to a channel length (L), a width (W), and a capacitance of the insulator of the organic thin film transistor shown in FIG. 3.
The on/off current ratio is defined by a ratio of current in a flow state and current in a cutoff state and an index of switching performance of the transistor. The threshold voltage is determined by a difference in work function of the gate and the organic semiconductor, an internal charge of a gate insulator, an interfacial charge, and the like. The sub-threshold slope is the magnitude of gate voltage required to increase the current 10 times at the threshold voltage and means an ability of controlling the interface of the organic semiconductor of the gate.
Many studies of electrode material, insulating layer material, organic semiconductor material have been made in order to improve the above-mentioned performance indexes. In connection with this, it is the most important and difficult to improve charge mobility of the organic semiconductor compound. The organic semiconductor compound has high crystallinity and the charge mobility thereof is increased as Π-orbit superposition is increased.
Many studies of substances and production processes have been made since the organic thin film transistor was reported by Tsumura et al. for the first time (A. Tsumura, K. Koezuka and T. Ando, Appl. Phys. Lett., 1986, 49, 1210). Organic substances such as low molecular weight substances, polymers, and oligomers have been studied to be used as semiconductor substances in the thin film transistor. As shown in the results of the studies, the performance of the organic thin film transistor is improved from 10−5□/Vs to 1□/Vs in views of charge carrier mobility of the thin film transistor (J. M. Shaw, P. F. Seidler, IBM J. Res. & Dev., 2001, Vol. 45, 3). The performance of the current organic thin film transistor is similar to that of an amorphous silicon transistor, and the organic thin film transistor is used in display apparatuses such as E-papers, smart cards, sensors, electronic tags (radio frequency identification, RFID), liquid crystal displays, and organic light emitting diodes.
There are two types of molecules that are capable of being used to constitute a semi-conductor layer and they are p-type and n-type organic semiconductor substances. In the p-type semiconductor substance, a hole is a charge carrier and, in the n-type semi-conductor substance, an electron is the charge carrier. Examples of the p-type organic semiconductor substance include pentacene, antradithiophene, benzodithiophene, thiophene oligomer, polythiophene, mixed-subunit thiophene oligomer, and oxy-functionalized thiophene oligomer (H. E. Katz et al., Acc. Chem. Res. 2001, 34, 359). Examples of the n-type organic semiconductor substance include fluorinated metallophthalocyanine (Z. Bao, J. Am. Chem. Soc. 1998, 120, 207) and perfluoroarene-modified polythiophene (A. Facchetti, Angew. Chem. Int. Ed. 2003, 42, 3900).
Currently, pentacene, oligothiophene derivatives, poly[3-hexyl thiophene], and the like are used as a material that has the desirable performance and is most extensively used. However, pentacene and the oligothiophene derivative has the desirable performance only in the case of when a thin film is formed by using a vacuum deposition process, but is problematic in that it is not easy to perform a solution process. The thin film may be formed by means of the soluble oligothiophene derivative and polythiophene derivative using the solution process such as screen-printing, ink-jet printing, micro-contact printing, spin coating, and dip coating. However, the charge mobility is still worse than that of the thin film transistor that is produced by using vacuum deposition of the low molecular system and the on/off current ratio (Ion/Ioff) is not enough to commercialize the organic thin film transistor.
Accordingly, in order to commercialize the organic thin film transistor, there remains a need to develop a low-priced organic semiconductor substance that is capable of being used during a solution process and has high charge mobility and on/off current ratio.